This invention relates generally to lubrication and lubricants, and more particularly to a hybrid lubricant in which solid lubricant particles are dispersed in a fluid lubricant carrier that includes a small but effective amount of a fluorochemical surfactant solution that acts to stabilize the dispersion and thereby prevent agglomeration of the particles.
Even the most carefully finished metal surfaces have minute projections and depressions therein which introduce resistance when one surface shifts relative to another. The application of a fluid lubricant to these surfaces reduces friction by interposing a film of oil therebetween, this being known as hydrodynamic lubrication. In a bearing, for example, the rotation of the journal causes oil to be drawn between it and the bearing so that the two metal surfaces are then separated by a very thin oil film. The degree of bearing friction depends on the viscosity of the oil, the speed of rotation and the load on the journal.
Should the journal start its rotation after a period of rest, it may not drag enough oil to float the surfaces apart; hence friction would then be considerably greater; the friction being independent of the viscosity of the lubricant and being related only to the load and to the "oiliness" property of the residual lubricant, causing it to stick tightly to the metal surfaces. This condition is referred to as "boundary lubrication," for the moving parts are then separated by a film of only molecular thickness. This may cause serious damage to overheated bearing surfaces.
The two most significant characteristics of a hydrodynamic lubricant are its viscosity and its viscosity index, the latter being the relationship between viscosity and temperature. The higher the index, the less viscosity will change with temperature. Fluid lubricants act not only to reduce friction, but also to extract heat developed within the machinery as well as a protection against corrosion.
Though fluid film separation of rubbing surfaces is the most desirable objective of lubrication, in practice it is often unobtainable. Thus bearings built for full fluid lubrication during most of their operating phases actually experience solid-to-solid contact when starting and stopping. Solid surfaces in rubbing contact are characterized by coefficients of friction varying between 0.04 (Teflon on steel) and &gt;100 (pure metals in vacuo). In contrast to fluid lubrication, solid lubrication is usually accompanied by wear of rubbing parts. Optical inspection of the surfaces after rubbing invariably reveals microscopic damage of the metal both when unlubricated and lubricated.
Typical solid lubricants are soft metals such as lead, layer lattice crystals such as graphite and molybdenum disulphide, and crystalline polymers such as "FLUON" (polytetrafluoroethylene or PTFE). Integral bonding of these solid lubricants to the surfaces of the bodies to be lubricated is desirable for good performance.
Under severe operating conditions usually encountered in automotive transmissions and in internal combustion engines, hydrodynamic or fluid lubrication is inadequate to minimize friction and wear; for fluid film separation of the rubbing surfaces is not possible throughout all phases of operation. Hence, the ideal lubricant for engines or other mechanisms having moving parts is one combining hydrodynamic with solid lubrication. In this way, when adequate separation exists between the rubbing surfaces, a protective fluid film is interposed therebetween; and when these surfaces are in physical contact with each other, friction therebetween is minimized by interposing solid lubricants between these surfaces.
In theory, one can best approach this ideal by lining the rubbing parts of engines with solid lubricant layers which are integrally bonded thereto, concurrent use being made of a lubricating oil which functions not only to provide hydrodynamic lubrication but also to cool the rubbing parts. In addition, the oil may carry synthetic organic chemicals to carry out other functions to counteract wear and prevent corrosion.
The practical difficulty with attaining this ideal is that parts coated with solid lubricants, such as a PTFE layer, are very expensive and therefore add considerably to the overall cost of the engine. Moreover, in PTFE-coated parts which operate under rigorous conditions, the solid lubricant layers bonded thereto have a relatively short working life, so that it is not long before the only lubricant which remains effective in the engine is the fluid lubricant.
In order to provide lubricating activity that has both solid and fluid components, my prior U.S. Pat. No. 4,127,491 discloses a modified oil lubricant suitable for an internal combustion engine provided with an oil filter as well as for many other applications which call for effective lubrication throughout all phases of operation. This modified lubricant is constituted by major amounts of a conventional lubricating oil intermingled with minor amounts of an aqueous dispersion of polytetrafluoroethylene particles in the sub-micronic range in combination with a neutralizing agent which stabilizes the dispersion to prevent agglomeration and coagulation of the particles. The modified lubricant is therefore capable of passing through the oil filter without separating the solid particles from the oil in which it is dispersed.
This modified lubricant has many significant advantages; for, as indicated in my prior patent, it reduces wear and thereby prolongs engine life; it makes possible a sharp reduction in the emission of pollutants and also effect a significant improvement in fuel economy, the last factor being of overriding importance in a fuel-short world.
To charge-neutralize and stabilize the PTFE dispersion and thereby prevent the colloidal particles from settling out, my prior patent adds a fluorochemical surfactant to the dispersion before it is intermingled with a lubricant carrier to create the additive. While the fluorochemical surfactants disclosed in my prior patent are generally effective for their intended purpose, we have found that long-term stability is not always attained; for after several months the modified lubricant additive, when stored in a container, may be subject to a slight but a nevertheless undesirable settling action.